An analysis of extracellular potentials from single neurons in the lateral geniculate nucleus of the cat.

The lateral geniculate nucleus of the cat was explored with micropipettes having submicroscopic tips. The only reliably recorded intracellular activity was from axons. Following orthodromic stimulation, the potentials recorded by the extracellular electrodes registered the net flow of current across the soma-dendritic membrane of the principal cell bodies. The current has three phases of flow away from the soma-dendritic membrane followed by a flow of current toward this membrane. The first component is ascribed to synaptic activity. Subsequent components are ascribed to the activity of the initial segment of the axon and a limited area of high threshold membrane on the soma. The evidence is interpreted as suggesting that most of the soma-dendritic membrane is excited synaptically to produce a postsynaptic potential, but is not excited electrically and does not produce a propagating spike.